Plant 215

Macadamia species (Proteaceae)

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Macadamia nuts

Unlike the myriad of foods that have been contributed to the modern human diet by the floras of the Americas, Europe and Asia, the Australian flora has contributed only one, the macadamia nut. Two of the four species in the rainforest genus Macadamia are the sources of edible seeds; one of the other species has toxic seeds that accumulate cyanide. Macadamia integrifolia is a small tree native to Queensland. In contrast, Macadamia tetraphylla is a tree that can reach 18 metres in height and is native to southern Queensland and northern New South Wales.

Macadamias have glossy, evergreen leaves that are arranged in whorls and produce pendulous spikes of small, attractive, insect-pollinated flowers. The fruits are hard, woody, indehiscent spheres, up to three centimetres in diameter, containing one or two seeds. The two edible species can be separated from each other based on flower and fruit characteristics (Macadamia integrifolia has white to pink flowers and rough fruits, whilst Macadamia integrifolia has white flowers and smooth fruits). These two species hybridise with each other in cultivation, the hybrid often being more productive than either of its parents.

Macadamias were introduced to Hawaii in the 1880s and were established as a commercial crop in the 1930s. Today, the vast majority of commercial macadamia cultivars, which are propagated through grafting, come from selections made as part of breeding programmes established in Hawaii. During the last century, macadamias also became a commercial crop in Mexico, particularly Macadamia tetraphylla. Macadamia trees start to become commercially productive after about seven years and can continue being productive for more than 100 years. Only in the late 1990s did Australia overtake Hawaii as a major producer of macadamia nuts, now second only to South Africa in annual production.

Macadamias are oil-rich seeds, the different species differing in their chemical characteristics. Macadamia tetraphylla seeds contain more sugar than Macadamia integrifolia seeds, and tend to char when roasted or baked. Consequently, most of the commercial macadamias used in cooking are from Macadamia integrifolia, whilst those that will be eaten raw are Macadamia tetraphylla.

Ironically, although now distributed in many parts of the world, macadamias are considered vulnerable in their native range.

The German-Australian botanist Ferdinand von Mueller used the name Macadamia to commemorate the 30-year-old Scottish-Australian chemist and politician John Macadam, 'the talented and deserving Secretary of our Institute', the Philosophical Institute of Victoria (now the Royal Society of Victoria).